An equilibrium mixture of xylenes contains roughly 24% para-xylene (PX), 56% meta-xylene (MX), and 20% ortho-xylene (OX). PX is relatively high value as compared with MX and OX, and it is desirable to isomerize OX and/or MX to PX. Vapor phase and liquid phase processes for isomerizing PX-lean streams to equilibrium for subsequent PX recovery are described in numerous patents. It is an active area of research.
A typical paraxylene production process involves the so-called xylene loop. An example is illustrated in FIG. 1, which is a simplified flow diagram showing three major operations that occur in the xylene loop. There are many vaporization and condensation steps.
Liquid feed, typically a C8+ aromatic feedstream which has previously been processed by known methods to remove C7− species (particularly benzene and toluene), is fed by conduit 1 to xylenes re-run 3, an apparatus per se well known in the art. The xylenes re-run (or more simply a fractionation column) vaporizes the feed and separates the C8 aromatics into an overhead mixture 5 of xylenes (OX, MX, and PX) and ethylbenzene (EB), and a bottom product 61 comprising C9+ aromatics. The overhead mixture typically has a composition of about 40-50% metaxylene (MX), 15-25% PX, 15-25% OX, and 10-20% EB. Unless otherwise noted herein, percentages are % weight. The overhead is then condensed in condenser 7, an apparatus also per se well-known in the art, and becomes the feed for the PX recovery unit 15, via conduit 9 and 13, a portion of the condensed overhead may be returned to re-run 3 as reflux via conduits 9 and 11.
The PX recovery unit 15 may employ crystallization technology, adsorption technology, or extraction technology, each per se well known in the art. These technologies separate PX from its isomers and are capable of producing high purity PX up to 99.9%, which is taken from unit 15 via conduit 17. Shown in FIG. 1 is the case where unit 15 is an adsorptive separation unit, such as a Parex™ Unit, in which case typically the extract 17, which comprises a desorbent, such as PDEB (paradiethylbenzene), needs to be separated, such as by distillation, from the desired extract PX in distillation column 19, which generates an overhead 23 that is condensed in condenser 25 to yield a liquid stream 27, which is a high purity PX stream. This stream 27 may be taken off via conduit 31 and optionally a portion may be returned to column 19 as reflux via conduit 29. The desorbent is returned to the PX recovery system 15 via conduit 21. Raffinate from the recovery system 15, comprising MX, OX, EB, and some PX, is removed via conduit 65 and sent to unit 37, discussed below. Note: a portion of raffinate in 65 may be recovered and marketed as low-value solvent xylene.
The raffinate 65, which comprises mainly MX, OX, EB, and desorbent is sent to fractionation column 37, generating overhead 33 and bottoms 63. Overhead 33 contains MX and OX, which is condensed in condenser 32 and sent via conduit 35 and then 41 to isomerization unit 43, discussed in more detail below. A portion may be returned to fractionator 37 via conduit 35 and then 39 as reflux. The desorbent in the bottoms product is returned to 15.
A stream consisting essentially of MX and OX and EB is sent to isomerization unit 43, an apparatus per se known in the art, to isomerize the MX and OX and optionally EB to PX. Conventionally unit 43 is a vapor phase isomerization unit. Conventionally there are one or more heat exchangers or furnaces associated with the system shown in FIG. 1 between the PX recovery unit 15 and the isomerization unit that are not shown for convenience of view. Likewise, hydrogen separators and hydrogen compressors are also not shown for convenience of view. These and other features, such as valves and the like, would be apparent to one of ordinary skill in the art in possession of the present invention.
The product of the isomerization unit 43 is sent via conduit 51 to the C7− distillation tower 53, which separates the product of isomerization into a bottom stream 59 comprising equilibrium xylenes and the overhead 47, comprising C7− aromatics, e.g., benzene and toluene. The overhead product is condensed in condenser 45 and then the distribution of liquid product via conduit 49 may be apportioned as desired between conduit 57 and conduit 55, the former of which may be disposed of in numerous ways which would be well-known per se in the art, and the latter conduit returning C7− aromatics as reflux to tower 53. The bottoms product 59 of distillation tower 53 is then sent to xylenes re-run 3, either merging with feed 1 as shown in the figure, or it may be introduced by a separate inlet (not shown).
Note that as used herein the term “raffinate” is used to mean the portion recovered from the PX recovery unit 15, whether the technology used is adsorptive separation, crystallization, or membrane, and then is sent to the isomerization unit 43, conventionally a vapor phase isomerization unit, which uses technology also per se well-known. The xylene isomerization unit (whether vapor phase or liquid phase) accomplishes two major things. It isomerizes the lower valued MX and OX to higher value PX and it also turns EB into benzene/toluene and light gases (so-called “EB destruction”) or optionally, isomerize EB to xylenes. EB destruction or EB isomerization prevents the build up of EB within the xylenes loop. Products from the isomerization unit are distilled to separate C7− compounds (particularly toluene and benzene) prior to being recycled back to the xylene re-run.
Particularly relevant patents include U.S. Pat. No. 6,689,929 U.S. Pat. No. 6,878,855; WO 2005/075389; and WO 2005/075390.
Recently the present inventor, along with others, has described with particularity processes involving the use, at least partially, of liquid phase isomerization in U.S. Provisional application Ser. Nos. 12/612,007 and 61/326,445
The present inventor has now discovered a process for PX production which in embodiments provides for a significant reduction in energy consumption by eliminating excessive vaporization and unnecessary recycling.